1. Field of the Invention
The present invention relates to a piercing/rolling method and a piercing/rolling apparatus which use a piercer employed in the Mannesmann tube making process that is a representative process of manufacturing seamless steel tubes.
2. Description of Related Art
Manufacture of seamless steel tube by the Mannesmann tube making process is generally carried out as follows; first, a hollow shell is obtained by passing a billet (round steel ingot) through a piercer and piercing the center of the billet, which is further rolled to elongate by means of a plug mill, for example, either as it is or after being processed in an elongating mill for explaining and rolling as required, then smoothed, corrected in the form and sized by means of a smoothing mill or a sizing mill, and subjected to a finishing process to obtain a seamless steel tube as the finished product. As the piercer and the elongating mill described above, a so-called inclined rolling mill is used which combines main rolls, of which axial line is inclined with respect to the pass line of a material to be rolled, and a plug.
The piercer used in the Mannesmann tube making process described above generally has a pair of main rolls disposed to oppose each other while interposing the specified pass line, a plug disposed along the pass line as an inner surface regulating tool and guide shoes or disk rolls disposed to oppose each other while interposing the pass line as tube guiding members.
FIG. 1 is a schematic plane view of the piercer used in the Mannesmann tube making process, and FIG. 2 is a cross sectional view taken along the line II--II of FIG. 1. In FIGS. 1, 2, numerals 21A, 21B denote the main rolls, numeral 2 denotes a plug used as the inner surface regulating tool, and numerals 31u, 31d denote disk rolls used as the tube guiding members. Letter B denotes the billet, namely the material to be rolled which is transferred in the direction indicated by the hollow arrow mark Y, then pierced and rolled to become a hollow shell H that is drawn out. Therefore the inlet side of the piercing process is located at the left and the outlet side of the piercing process is located at the right in FIG. 1. FIG. 2 shows the cross section viewed from the inlet side of the piercing process.
As shown in FIG. 1, the main rolls 21A, 21B have gorge portions 41 near the middle portion in the axial direction thereof where each roll has the maximum diameter, the gorge portion being interposed by two conical portions of diameters gradually reducing toward the one conical portion ends having an inlet face 42 of inlet face angle .theta.1 and the other conical portion having an outlet face 43 of outlet face angle .theta.2, thereby forming an overall configuration of barrel type, while the main rolls 21A, 21B are disposed to oppose each other on the right and left or over and below the pass line X--X of the billet B. The main rolls 21A, 21B are driven by electric motors not shown in the drawing.
The plug 2 has an overall configuration of a bullet, with the base end thereof being supported on the tip of a mandrel bar M. The plug 2 is positioned on the pass line X--X to be held thereon at the middle of space between the main rolls 21A, 21B, and is capable of rotating about the pass line X--X. The base end of the mandrel bar M is linked to a thrust block which is not shown in the drawing.
As shown in FIG. 2, the disk rolls 31u, 31d have disk forms having concave surfaces on the periphery that face the plug 2, and are disposed to oppose each other on the right and left or over and below the pass line X--X in such a configuration as arranged alternately with the main rolls 21A, 21B. The disk rolls 31u, 31d are driven by electric motors not shown in the drawing.
In the piercer of such a configuration as described above, when the main rolls 21A, 21B are driven to rotate in the directions of the arrow marks of FIG. 2, the billet B being transferred in the direction Y along the pass line X--X is engaged between the inlet faces 42, 42 of the main rolls 21A, 21B thereby to be pierced while rotating clockwise around the pass line X--X when viewed from the inlet side of the piercing process. Thus the billet B is pierced by the plug 2 while being pressed on both sides by the gorge portions 41 of the main rolls 21A, 21B, to become the hollow shell H which is drawn out.
Techniques to carry out the piercing/rolling operation with the disk rolls being inclined with respect to the pass line in such a direction that improves the piercing efficiency of the material in the piercer employing disk rolls have been disclosed, for example, in the Japanese Patent Application Publication No. 59-47605 (1984).
The deformation form of the material in the course of piercing at the outlet side of the piercing process from the gorge portion of the main roll in the piercing/rolling operation of the prior art is shown in FIG. 3 which is contained in a published document (STAL IN ENGLISH, AUGUST, 1970, pp. 632-635), showing that bulging of the outer diameter of the material on the side that is engaged by one of the main rolls is larger than that of the material on the side leaving the other main roll. This is explained as follows with reference to FIG. 4 that shows an example of configuration provided with disk rolls. The bulging of the portion (part B in drawing) that is being engaged by the main roll 21B is larger than the bulging of the portion (part A in drawing) that is leaving the main roll 21A. Therefore, a pair of tube guiding members for suppressing the bulging of the outer diameter of the material are usually provided between the main rolls 21A and 21B, above and below the pass line in the case shown in FIG. 4.
In an ordinary piercing/rolling process wherein the expansion ratio of the outer diameter (ratio of the outer diameter of the material after piercing to the outer diameter of the material before piercing, namely the material outer diameter after piercing divided by the material outer diameter before piercing) is in a range from 1.0 to 1.05, the bulging of the outer diameter of the material at part B shown in FIG. 4 does not pose a problem. When the expansion ratio of the outer diameter increases, however, the circumferential length of the material at the outlet side of the main roll increases and therefore the bulging at part B becomes greater than the bulging at part A of FIG. 4, thereby making the contact angle .phi. of the main roll 21B greater than in the case of ordinary piercing/rolling operation. As a result, when a non-stationary piercing/rolling operation (piercing/rolling with high expansion ratio of outer diameter) is carried out wherein the thrusting force in the rolling direction becomes smaller, problems occur such as the incomplete release from the main rolls caused by the inability of the material to rotate during rolling the bottom of the tube material, the form of the bottom portion of the hollow shell becoming elliptical in configuration and the shoe mark flaws generated on the outer surface of the hollow shell.
Piercers configured by disposing disk rolls in an inclined arrangement in order to prevent the material from being squeezed into the space between the disk rolls and the main rolls are disclosed, for example, in the Japanese Patent Application Laid-Open No. 63-90306 (1988).
While the piercer disclosed in the Japanese Patent Application Publication No. 59-47605 (1984) employs such a configuration as a disk roll used on one side and a fixed guide used on the other side, the piercing speed is increased in case disk rolls are used on both sides and are inclined toward the main roll 21B as shown in FIG. 5. However, increasing the expansion ratio of the outer diameter leads to increased resistance against the material near the edge of the outlet side thereof in the sliding face of the disk roll, resulting in such problems as seizure of the edge or damage on the circumferential surface of the material at the edge causing shoe mark flaws to remain on the product, and stoppage of the piercing process due to increased contact angle .phi. of the main rolls in the rotating direction of the material.